There is a large market for light olefin products ethylene and propylene which are, for example, used in the production of plastics. Light olefins are traditionally produced via the cracking of petroleum feedstocks, e.g. through catalytic cracking, steam cracking, or some combination of the two processes. However, as petroleum feedstocks from crude oil face increasing prices, other sources of ethylene and propylene are becoming more attractive.
It is known to produce olefins from oxygenates via an oxygenate to olefins (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can, for example, be converted to methanol or dimethyl ether (DME), both of which are suitable oxygenate feedstocks for an OTO process. Oxygenates can also be produced from biomass.
In an OTO process, an oxygenate such as methanol is provided to a reaction zone comprising a suitable conversion catalyst, and converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the methanol is converted to higher hydrocarbons including C4+ olefins.
The catalyst is usually a molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 describes a process for making olefins from methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. U.S. Pat. No. 6,797,851 describes a process for making ethylene and propylene from an oxygenate feed using two or more zeolite catalysts.
OTO processes can encounter problems due to the catalyst being fouled, for example due to coke deposition on the catalyst. Coke herein refers to a carbonaceous composition which next to the prevailing carbon may also contain hydrogen and other elements. Conventional catalyst regeneration techniques can be employed to remove the coke.
It has now been realised that the purity of the oxygenate feed may also affect the viability of the catalyst. In particular, the catalyst appears to be susceptible to deactivation by contaminants in the oxygenate feedstream such as metals.
US 2011/014404 describes a process for improving the quality of an oxygenate feedstream to an oxygenate to olefins conversion reactor by passing the feedstream to a feed fractionation column. This process therefore involves distilling the feedstream, in order to separate the oxygenate from undesired impurities such as sodium hydroxide (NaOH).
It is an aim of the invention to provide an improved oxygenate to olefins (OTO) process.